i don't have any One Weird Trick for it, I just try to be methodical: once the melody is sketched out, I figure out a key area that fits and make my own chords that logically fit underneath. it might be totally off but as long as you're finding cadences and modulations you're getting pretty close. once you're down in the trenches of figuring out each chord change there's a lot of trial and error (and as a pianist I can imagine that's easier for you? but I totally get that it's tedious and kind of frustrating)

and ultimately, if you've gotten close but not quite perfect.. that's okay, it's your cover, it can do whatever you like - just having a full, if imperfect, cover to listen to can really help you hone in using your ear training to find what doesn't quite mesh and then edit those sections

it's a process though! so just doing any one of these steps on a given day would be awesome, thinking too hard about the End Product really chokes me up especially in this time of uh nothingness tbh